The size (gauge) of the wires are relatively huge and really don't come into play when talking of Power drawn (watts) from the card it self. Your 1500W microwave pulls off a pair of wires of pretty much the same gauge compared to the SIX or TWELVE going into the graphics card. Gauge is usually only increased when talking about amperage. Watts simply aren't exerted on the wiring.
There's a few things with this part of the post though. The most amperage that 1500W microwave is going to pull is 12.5A @ 120V. If you're having one one PCI-E plug pull at least 150W, at 12V, this is 12.5A. Also, since the microwave wire is only running AC electricity, it puts a lot less resistance than DC electricity, which a PCI-E plug carries.
Either way, the specs I pulled out were for the terminals, not the wires.